In the radio-frequency (RF) transmission architectures of modern wireless communication systems, there are two mainstreams of receiver architectures that are highly integrated as well as providing multiple modes. One is the low intermediate frequency (IF) receiver; the other is the direct conversion, or zero IF, receiver. These two types of receivers own complementary pros and cons, and are extensively valued and applied in the industry. The former receiver can avoid problems of DC shifts and low-frequency noises. However, it suffers from interferences by image signals. On the contrary, the latter receiver has little interferences by image signals. Nevertheless, it has the problems of DC shifts and low-frequency noises.
At present, low- and intermediate-frequency architectures are widely applied to transmitting and receiving sides of wireless communication. Thereby, the problems of image interference and circuit areas in low- and intermediate-frequency receiver architectures have become a major issue in the industry as well as in the academia. The most popular method is, in a low- and intermediate-frequency or an ultra-low- and intermediate-frequency receiver architecture, to use a mixing circuit to down-convert the wireless RF signal received by an antenna and output a pair of orthogonal signals. A complex filter architecture is then used for processing the pair of signals. Starting from the output of a low noise amplifier (LNA) in a general image rejection receiver architecture, passing mixers, filters, amplifiers, analog-to-digital converters, and till the output of a baseband processing circuit, signals are divided into two phases, namely, 0-degree (in-phase, I) and 90-degree (quadrature, Q) signals. Consequently, two sets of mixers, filters, amplifiers, and analog-to-digital converters are required, hence consuming more circuit area and power consumption as well as widening asymmetry between I and Q signals.
Accordingly, the present invention provides a novel receiving apparatus of a communication system, which can avoid using two sets of amplifiers and analog-to-digital converters. Thereby, circuit area and power consumption can be reduced, and the problems described above can be solved.